TypeError: Failed to fetch dynamically imported module

[IPWright83]

Describe the bug

Since switching to Vite we noticed a new production issue, where sometimes users are encountering an error if we deploy while they have an active session:

TypeError: Failed to fetch dynamically imported module

I believe this is because if any code is modified in an area that Vite would turn into a dynamic module, then the file hash changes, however when they try to visit an area that would trigger the dynamic load, those files no longer exist so they hit the error message above.

Quoting from https://stackoverflow.com/a/74057337/21061

When you dynamically import a route/component, during build it creates a separate chunk. By default, chunk filenames are hashed according to their content – Overview.abc123.js. If you don't change the component code, the hash remains the same. If the component code changes, the hash changes too - Overview.32ab1c.js. This is great for caching.

Now this is what happens when you get this error:

• You deploy the application
• Your Home chunk has a link to /overview route, which would load Overview.abc123.js
• Client visits your site
• You make changes in your code, not necessarily to the Overview component itself, but maybe to some children components that Overview imports.
• You deploy changes, and Overview is built with a different hash now - Overview.32ab1c.js
• Client clicks on /overview link - gets the Failed to fetch dynamically imported module error, because Overview.abc123.js no longer exists

That is why the errors correlate with deployments. One way to fix it is to not use lazy loaded routes, but that's not a great solution when you have many heavy routes - it will make your main bundle large

What I expect to happen, is not to encounter any errors if the users session remains active during a deployment.

I have been unable to come up with a good workaround (specifically for me using React ErrorBoundary is the best I can do so far with a re-direct similar to https://stackoverflow.com/a/74861436/21061 which is a mitigation and provides quite a poor user experience flashing an error message).

Reproduction

https://github.com/IPWright83/vite-dynamic-import

Steps to reproduce

The above repository has been set up to mimick a production deployment as obviously that is a much more complicated set-up. It leverages React.lazy to force a dynamic module and uses a setTimeout to provide a delay with which to simulate a user navigation to a page requiring a module. In a real production scenario I don't believe React.lazy is required.

• Clone the above repository

• npm install

• npm run build

• open a 2nd terminal

• Terminal 1 npx serve dist (starts a web server)

• Browser open the URL (usually localhost:3000)

• Text Editor modify src/Foo.jsx changing the string "Foo" to something else (within 30s of launching the page - increase the setTimeout in App.jsx if this is not long enough)

• Terminal 2 npm run build

Wait... 30s after loading the page you should see a blank page render with errors in the browser console:

If you were to reload the page, you can see that Foo-b53985a6.js has been renamed to Foo-535d5a10.js (or similar new hash)

System Info

❯ npx envinfo --system --npmPackages '{vite,@vitejs/*}' --binaries --browsers

  System:
    OS: Linux 5.15 Ubuntu 20.04.5 LTS (Focal Fossa)
    CPU: (12) x64 Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz
    Memory: 18.36 GB / 31.10 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 16.14.0 - ~/.nvm/versions/node/v16.14.0/bin/node
    Yarn: 1.22.18 - ~/.nvm/versions/node/v16.14.0/bin/yarn
    npm: 8.3.1 - ~/.nvm/versions/node/v16.14.0/bin/npm
  Browsers:
    Chrome: 109.0.5414.74
    Chromium: 109.0.5414.74
    Firefox: 109.0

### Used Package Manager

pnpm

### Logs

_No response_

### Validations

- [X] Follow our [Code of Conduct](https://github.com/vitejs/vite/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [Contributing Guidelines](https://github.com/vitejs/vite/blob/main/CONTRIBUTING.md).
- [X] Read the [docs](https://vitejs.dev/guide).
- [X] Check that there isn't [already an issue](https://github.com/vitejs/vite/issues) that reports the same bug to avoid creating a duplicate.
- [X] Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to [vuejs/core](https://github.com/vuejs/core) instead.
- [X] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/vitejs/vite/discussions) or join our [Discord Chat Server](https://chat.vitejs.dev/).
- [X] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.

[sapphi-red]

This is not something specific to Vite and happens with Webpack, too.

https://mitchgavan.com/code-splitting-react-safely/ http://dimaip.github.io/2020/04/25/deploying-apps-with-code-splitting/

I don't have any idea to solve this on a bundler side.

[awacode21]

We are experiencing the same issues with a vite+vue SSR application. we are experiencing thousands of those a day. also multiple days after deployment... we also saw this after our very first go-live, where no "previous version" was online. We are completely desperate since one week on how to fix this. Other strange thing is, sometimes we get this error logged in Sentry, and when we try to check if the file really is not available, we are able to access it. Others are not working

[IPWright83]

Thanks @sapphi-red, is it Vite or Rollup generating these hashes (assuming that's what they are) on the filenames? I wonder if there's a way to suppress that behaviour and if it's required for cache busting whether there's any alternative approaches.

[victorlmneves]

@sapphi-red both generate hashes when running the build. If your product it's a SPA you can try to use a service worker together with an interceptor (if you use Axios) to manage that. I did it on Vue 2 where it's basically stores on the localStorage a version of the app and then compares if the one that you access it's the same or not and if not it triggers a reload of the browser in order to get the updated version

[IPWright83]

@victorlmneves could you provide a bit of a fuller explanation/ code snippets maybe of tha so I understand the concepts a bit more? I'd be quite interested in exploring what that might look like.

[victorlmneves]

@IPWright83 https://dev.to/voodu/vue-3-pwa-service-worker-12di https://medium.com/@FezVrasta/service-worker-updates-and-error-handling-with-react-1a3730800e6a

And here is an example that I did some years ago

import { register } from "register-service-worker"

if (process.env.NODE_ENV === "production") {
  register(`${process.env.BASE_URL}service-worker.js`, {
    ready() {
      console.log(
        "App is being served from cache by a service worker.\n" +
          "For more details, visit https://goo.gl/AFskqB"
      )
    },
    registered() {
      console.log("Service worker has been registered.")
    },
    cached() {
      console.log("Content has been cached for offline use.")
    },
    updatefound() {
      console.log("New content is downloading.")
    },
    updated() {
      console.log("New content is available please refresh.")
      caches.keys().then(names => {
        for (let name of names) {
          console.log(name)
          caches.delete(name)
        }
      })
      setTimeout(() => {
        window.location.reload()
      }, 1000)
    },
    offline() {
      console.log(
        "No internet connection found. App is running in offline mode."
      )
    },
    error(error) {
      console.error("Error during service worker registration:", error)
    }
  })
}

And another one using Axios interceptors

axios.interceptors.response.use((resp) => {
  let v = resp.headers['vers'] || 'default'

  if (v !== localStorage.getItem('vers') && resp.config.method == 'get') {
    localStorage.setItem('vers', v)
    window.location.reload() // For new version, simply reload on any get
  }

  return Promise.resolve(resp)
})

[sapphi-red]

@IPWright83 You can remove these hashes from the filename by configuring entryFileNames and chunkFileNames.

import { defineConfig } from 'vite'

export default defineConfig({
  build: {
    rollupOptions: {
      entryFileNames: '[name].js',
      chunkFileNames: '[name].js'
    }
  }
})

But yes this is recommended for cache busting.

I don't know any alternatives other than the ones described in the articles I linked above.

[victorlmneves]

@IPWright83 by the way, have you tried to access directly the file that is displayed on the console that wasn't possible to import? I'm asking because on our project we get those errors randomly but when we access them directly the file exist

[IPWright83]

I have @victorlmneves. Is there any way to adopt an approach more like this https://github.com/vitejs/vite/issues/5825 @sapphi-red? Anything after the queryString can still cache-bust, but as the file name is constant you'll get the new payload. I think that example is a little primitive (I think JS needs transforming too). Could you see any issues with this approach?

[sapphi-red]

I guess it's possbile by using a plugin. I think that will lead to runtime errors sometimes because the dynamic import in old version files can resolve to new version files.

For example, first version deployed

// foo.js
export const foo = 'foo'
export const bar = 'bar'

// a file depends on foo.js
import('./foo.js').then(mod => {
  if (mod.bar.startsWith('b')) {
    console.log(mod.foo)
  }
})

second version deployed

// foo.js
export const foo = 'foo'

// a file depends on foo.js
import('./foo.js').then(mod => {
  console.log(mod.foo)
})

[victorlmneves]

@IPWright83 Meanwhile, I also saw someone handling the problem this way

router.onError((error, to) => {
  if (error.message.includes('Failed to fetch dynamically imported module')) {
    window.location = to.fullPath
  }
})

[IPWright83]

@victorlmneves yeah, that works for vue-router and is in the StackOverflow link I shared. Unfortunately we're using react-router through various indirection connected-react-router & react-router-config. So even if it did expose an onError handler which I've been unable to find, we'd struggle calling it :(

[mikemklee]

Any updates on this? Also running into the same issue with a vue 2 + vite app, when using dynamic imports.

The vue router hack could work for some imports, but we have dynamic imports outside of the router, which i don't think the hack would be able to cover.

Besides, forcing a reload because an error occurred doesn't seem like the best UX, so wondering if there's a solution/workaround that is more subtle and "behind the scenes" 🤔

[yakobe]

While I understand that we should try to solve this problem ourselves, it seems like this must be quite a fundamental problem that developers will have when creating apps with vite? Especially when continuous deployment comes into the mix, because the files update very often and trigger lots of errors. For now I am back to "nightly deployments" to try and minimize disruption 😬.

It would be great if there was a standard solution somewhere in the docs. Preferably one without making the users manually "opt-in" to a page reload. Even nicer would be if vite could somehow handle it for us so we don't have to do anything. Eg catch the error and refresh the files without a hard reload of the page. That would greatly improve the DX 🙂.

Thanks for all the great work. Vite is awesome.

[victorlmneves]

@yakobe in my case, it's not about outdated chunks. I'm getting this with existing files that is even worst to figure out the problem and try to fix it, especially because the only way I'm able to reproduce the issue it's shutting down the node server (I have SSR app) :/

[lzacharkow]

For what it's worth, an ad-blocker was the root of this issue for me. Disabling it resolved my issue.

[lmiller1990]

A lot of Cypress users run into this: https://github.com/cypress-io/cypress/issues/25913

I don't think I can fix it on the Cypress end. I can spend time (several days of development resources) to patch Vite if needed, but it looks like the cause isn't entirely clear.

Some observations:

1. Happens mostly with large bundles - like MUI (Material UI).
2. Especially happens on CI with Docker + many instances running (eg, race condition/resource issue)?

Any tips -- maybe from someone more familiar with Vite - on debugging this?

I will inspect Cypress more first and keep the limitations in mind.

[benadamstyles]

We are just hitting this now too, but it's not just when the user has a browser tab open. It can also happen much later, if the user returns to our app after we have deployed. We're trying to work out if it's caused by the index.html page being cached for too long, rather than an issue with the JS file hashes themselves. Maybe it helps someone who is experiencing this? My reasoning goes like this:

• Let's say you have index.html which references index.abc.js which dynamically imports moduleX.123.js.
• You update your code and deploy it, so now you have index.def.js which dynamically imports moduleX.456.js. This should not cause any "module not found" issues because all the file hashes are new, even the entry index JS file.
• However, if your index.html file has been cached for longer, it still contains the reference to index.abc.js, which dynamically imports the now deleted moduleX.123.js. Hence you get an error.

I'm still confirming this but I wondered if it matches anyone else's experience/setup?

[AlfonsoUceda]

Happened to me after upgrading from vite 4.0.1 to 4.1.4

[adamkpurdy]

@benadamstyles I had the same issue. I removed the hashing option in the config like another user mentioned.

[aakash14goplani]

I am facing same issue with SvelteKit + Vite.

[eifr-vault]

Not sure if related but I'm getting the same error when using storybook with a react component that imports a module from an index.ts file (exports all others)

[namu254]

ad-blocker

same

[lmiller1990]

I reproduced this in CI only, you can follow my progress as I debug it in Cypress: https://github.com/cypress-io/cypress/issues/25913#issuecomment-1460983617 I suspect the same core issue. I think it's a race condition or resources related issue.

[aethr]

I think there are possibly two separate issues being described by different users/teams in this thread:

1. failed to fetch after a new deployment where the old version was deleted from the server

• in this case, attempting to load the resource referenced by the error is expected to fail, it has been deleted

2. failed to fetch that occurs somewhat randomly

• in this case, attempting to load the resource succeeds

For people experiencing issue 1., the solution is to organise your deployments so that old versions aren't deleted immediately. Doing a deployment of new app code doesn't mean that all users running your app in their browser will update immediately (unless you force it somehow).

For those users who still have an older version running in their browser (lets say index.123.js), navigating to routes that load chunks dynamically will always reference the chunk that was current when they loaded your app (ie home.123.js), and this is correct. You don't want the user getting half of their chunks with new app code and half with old, this would lead to very hard to debug issues. If you delete those chunks while the user is still navigating around the app, you will hit this error when they change routes and try to load a chunk they haven't loaded yet.

For users that load the app after a deployment, index.html will reference a new index.456.js chunk that references all the other chunks that are consistent with that version (ie home.456.js if there are also changes in that chunk). The only difference is which version was referenced by index.html when the user loaded the tab.

If you keep a manifest of each deployment, and your instrumentation/logging includes the "version" of the app users are using, you should be able to figure out how long old versions are still used. My team has seen users still operating in the same tab they loaded a month ago without updating, so we keep all our built assets deployed for a long time. If you want to force users to reload when a new version is deployed, that's a whole piece of engineering in itself.

[lmiller1990]

According to quite a bit of research in https://github.com/cypress-io/cypress/issues/25913 at least one class of error is related to CI on GH Actions specifically - not sure if this helps anyone, but worth noting in case it does.

[victorlmneves]

@lmiller1990 according to some of the descriptions I saw on https://github.com/cypress-io/cypress/issues/25913 it looks more or less like the problem that I have where we aren't able to reproduce it locally. But most likely, my case is a different scenario from the others as it's an SSR + SPA and the dynamics imports are falling for existing chunks (we also have a few for no longer existing chunks after a deployment, but not worried based on the number of occurrences) As final notes, not sure if this can help to have more clues on the problem, locally as I said we never got those errors...unless we kill/stop the local server when the page is almost rendered on the browser, and that way we get the errors. On the other hand on the defineAsyncComponent I have tried this approach without success as it was executing the request 3 more times and even so, we got the error as the end it still failed the import

import { defineAsyncComponent } from 'vue';

export const useComponent = (name: string) => {
    return defineAsyncComponent({
        loader: () => import(`../components/${name}/${name}.vue`),
        onError(error, retry, fail, attempts) {
            if (error.message.match(/fetch/) && attempts <= 3) {
                retry();
            } else {
                fail();
            }
        }
    });
};

[am-maneaters]

i'm getting this error when i deploy my site to github pages. however, when i deploy the same built code to netlify, it works just fine.

[mikemklee]

i'm getting this error when i deploy my site to github pages. however, when i deploy the same built code to netlify, it works just fine.

Similarly, running into issues when app is deployed to Cloudflare CDN. Also did not have this issue when app was deployed to Netlify.

I wonder if there are any differences in cache invalidation behavior between CDNs, and whether they are contributing to the issue?

[tkayo]

just adding onto this issue where we are running vite ^4.0.4 for local development we see this error happening quite often as we use defineAsyncComponent throughout our application. We've tried combinations of both checking and uncheckingdisable cache in the browser (chrome inspector), dumping the vite cache, and just restarting vite to get by the error.

the most stable option seems to be running in icognito mode for the browser but not quite sure why this works. seen mentions of adblocker in the responses here but some of us do not even have adblocker installed. it would be great to not have to run in icognito mode to resolve this issue